The contrast detection technique (also called the “blur technique”) and the phase difference detection technique (also called the “skew technique”) are known as general techniques for auto focus detection/adjustment methods in image capturing apparatuses using light beams that have passed through an imaging lens. The contrast detection technique is a technique widely used in video movie devices that capture moving images (camcorders), digital still cameras, and so on, and in such a case, the image sensor thereof is used as a focus detection sensor. This technique focuses on the signal outputted from the image sensor, and particularly information of the high-frequency components (contrast information) thereof, and uses the position of the imaging lens where the evaluation value of that high-frequency component information is the highest as the in-focus position. However, as implied by the name “hill-climbing technique”, the evaluation value is found while minutely moving the imaging lens, and it is ultimately necessary to move the lens to where the maximum evaluation value can be detected; this technique is therefore unsuited to quick focus adjustments.
The other technique, which is the phase difference detection technique, is often employed in single-lens reflex cameras that use silver film, and is the technique that has contributed the most to the practical application of auto focus (AF) detection in single-lens reflex cameras. With the phase difference detection technique, a light beam passing through the exit pupil of the imaging lens is divided into two parts, and the resulting light beams are received by a pair of focus detection sensors; the amount of skew in signals outputted based on the amount of light received, or in other words, the amount of relative positional skew in the division direction of the light beam, is detected. As a result, the amount of skew in the focus direction of the imaging lens is detected directly. Accordingly, the amount and direction of the focus skew can be obtained by performing a single accumulation operation using the focus detection sensor, thus making fast focus adjustment operations possible. However, in order to divide the light beam that has passed through the exit pupil of the imaging lens into two parts and obtain signals corresponding to the resulting light beams, a means for dividing the optical path, such as a quick return mirror, a half mirror, or the like, is generally provided in the optical path for imaging, and a focus detection optical system and AF sensor are generally provided at the end thereof. This is disadvantageous in that it increases the size and cost of the apparatus.
In order to circumvent the aforementioned disadvantage, a technique has been proposed in which an image sensor is provided with phase difference detection functionality in order to eliminate the necessity for a dedicated AF sensor and realize high-speed phase difference AF.
For example, in Japanese Patent Laid-Open No. 2000-156823 (hereinafter, “Patent Document 1”), a pupil division function is provided by offsetting the sensitive region of the light-receiving portion relative to the optical axis of an on-chip microlens in some light-receiving elements (pixels) in an image sensor. A configuration that performs phase difference focus detection is realized by using those pixels as focus detection pixels and disposing the focus detection pixels at predetermined intervals in groups of pixels used for imaging. Because the areas in which the focus detection pixels are arranged correspond to areas in which imaging pixels are absent, image information is produced through interpolation using the information from peripheral imaging pixels. In addition, when shooting moving images, thinning is executed while reading out from the image sensor, but in the case where a certain frame rate is demanded, as with moving images, the production of image information by compensating for losses caused by the focus detection pixels is too slow, and therefore the focus detection pixels are arranged in a row that is not read out during this thinning readout.
Meanwhile, Japanese Patent Laid-Open No. 2003-189183 (hereinafter, “Patent Document 2”) discloses an image capturing apparatus capable of switching between a thinning readout mode and an adding readout mode for output with the goal of improving the image quality of moving images and improving the sensitivity at low luminosities. In other words, Patent Document 2 proposes improving the image quality of moving images by performing readout in the adding mode in order to reduce moirés when the object has a high spatial frequency and the occurrence of moirés can be foreseen, or using the thinning readout mode in the case where the luminosity is high and the occurrence of smearing can be foreseen.
In addition, in Japanese Patent Laid-Open No. 2008-85535 (hereinafter, “Patent Document 3”), a pupil division function is provided by offsetting the sensitive region of the light-receiving portion relative to the optical axis of an on-chip microlens in some light-receiving elements (pixels) in an image sensor, in the same manner as in Patent Document 1. A configuration that performs phase difference focus detection is realized by using those pixels as focus detection pixels and arranging the focus detection pixels at predetermined intervals in groups of pixels used for imaging. Patent Document 3 also proposes taking accumulation control signals from the image sensing pixel groups and the focus detection pixel groups independently and employing different accumulation times for the two pixel groups, thereby improving the frame rate of the captured image and improving the performance of the focus detection pixel group with respect to low-luminosity objects.
However, the aforementioned known techniques have problems such as those described hereinafter.
With the technique disclosed in Patent Document 1, there are three types of readout modes: a still image mode that reads out all the pixels; a thinning readout mode that performs thinning so as to read out only the rows in which imaging pixel groups are present; and a ranging readout mode that reads out only the focus detection pixel groups. For this reason, the focus detection pixels are not read out when using the electronic viewfinder, when in a moving image mode, and so on, and thus while the frame rate of moving images can be improved, there is a problem in high-speed focus detection using the phase difference technique is impossible while a moving picture is being displayed.
The invention disclosed in Patent Document 2 relates to switching between a thinning readout mode and an adding readout mode depending on the scene when performing readouts for moving images. Focus detection pixels are not arranged in the image sensor, and thus using some of the pixels in the image sensor to perform phase difference focus detection is not considered from the outset. Even if, for example, focus detection pixels were present, those focus detection pixels could not be used for image information due to the reasons described earlier, and thus during the adding readout mode, the focus detection pixels would not be able to be added to the image capturing pixels. Furthermore, if an attempt was made to perform focus detection using the focus detection pixels, it would be necessary to read out the focus detection pixels singly even during the adding readout mode.
The invention disclosed in Patent Document 3 employs a configuration in which accumulation control signals from image sensing pixel groups and focus detection pixel groups are taken independently and the optimal accumulation times for the respective pixel groups can be set, thereby balancing image display refresh capabilities with rangefinding capabilities for low-luminosity objects. However, this is problematic in that the number of signal wires arranged between pixels increases, leading to a drop in the numerical aperture of the pixels and a drop in the sensitivity thereof. Meanwhile, Patent Document 3 discloses, as a variation on the invention described therein, commonalizing the accumulation control signals between the image sensing pixels and the focus detection pixels. This is advantageous in that the wiring between the image sensing pixels and the focus detection pixels is reduced, thereby improving the numerical aperture. However, this also means that the accumulation control is the same for both the image sensing pixels and the focus detection pixels. Therefore, Patent Document 3 discloses adding the output of the focus detection pixel groups multiple times in order to improve the S/N ratio of the focus detection pixels. However, this is problematic because even if that output is added following the readout, noise from a pixel amplifier, a readout gain amplifier, or the like is added multiple times as well, and thus the S/N ratio is not improved as in the case of accumulation time control.